The present invention relates generally to a transmissive body of high density polycrystalline alumina exhibiting improved total optical transmission as well as in-line transmission. A tubular form of the optically improved material when used as the light transmissive envelope for the high intensity electric discharge lamp provides higher light output than can generally be obtained with conventional polycrystalline alumina material. Such improvement is attributable to uniformity in the size and shape of the individual alumina grains along with substantial absence of pores and any secondary phase at the grain boundaries in said material.
The polycrystalline alumina material made in accordance with U.S. Pat. No. 3,026,210--Coble, assigned to the assignee of the present invention, has proven generally useful for the light transmissive envelope in high intensity electric discharge lamps. This polycrystalline alumina material is characterized by relatively uniform large grain size and can be prepared with a minimum of secondary phase magnesia-alumina spinel at the grain boundaries in order to provide optimum in-line transmission. A number of further modified polycrystalline alumina materials are also known which are said to exhibit improved in-line transmission attributable to either an addition of various grain-growth inhibiting agents in the powdered alumina mixture along with magnesia or otherwise varying the method of preparation. For example, a uniform grain structure of reduced size is said to be achieved in U.S. Pat. Nos. 3,711,585 and 3,792,142 by adding lanthana and/or yttria along with magnesia to the pure alumina powder to provide combined in-line transmission improvement and greater mechanical strength. A different approach which does not include any addition of grain-growth inhibiting oxides other than magnesia to achieve the same kind of improvement is described in U.S. Pat. No. 3,311,482 where a small size and uniform grain structure is said to be obtained by modifying the sintering conditions. All said variations do not eliminate secondary phase in the final sintered product, however, which contributes to reduced in-line transmission by reason of differences in refractive index between alumina and the secondary phase material.
It has further long been recognized in U.S. Pat. No. 3,026,177, assigned to the present assignee, that residual pores in the final sintered product must also be suppressed for optimum in-line transmission. A recent investigation finds that a pore volume fraction as small as 10.sup.-2 -10.sup.-3 can be primarily responsible for light scattering in polycrystalline alumina material and thereby have more of a detrimental effect than either grain boundary or secondary phase scattering. The means utilized to reduce porosity in accordance with this patented method of preparation features no use of a grain growth inhibiting additive in the powdered alumina starting material in combination with a two-stage sintering technique said to enhance removal of residual trapped pores. Such necessity for double sintering under special conditions and the attendant costs involved, however, has not lead to significant commercialization of said method.
The light output or luminous output of high intensity electric discharge lamps, especially sodium vapor lamps, depends upon the optical transmission of the light transmissive envelope wherein the arc discharge is generated. More particularly, the in-line transmission characteristics of said envelope are especially critical since passage of the emitted radiation without internal reflection has important advantages. Internal reflection of the generated radiation within said lamp envelope can result in significant absorption of the reflected radiation by the arc discharge. Passage of the generated radiation through the lamp envelope walls without reflection also affects heat flow and temperature distribution for the lamp in a desirable manner. By minimizing such internal reflection for improved in-line transmission, it has been found that the lamp envelope walls run significantly cooler which can permit lamp redesign to run the arc discharge at higher temperatures for both greater efficiency and a more desirable whiter color of lamp emission. Consequently, there is a continuing need to provide still greater in-line transmission for polycrystalline alumina material and in a manner which does not require costly modification of existing commercial manufacture.